Bleaching of wood pulps with reductive bleaching agents and thiourea



United States Patent 3,507,743 BLEACHING 0F WOOD PULPS WITH RE- DUCTIVE BLEACHING AGENTS AND THIOUREA Louis P. Cartsunis, Morrisville, Pa., and Robert R. Kindron, Pennington, N.J., assignors to FMC Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Feb. 23, 1965, Ser. No. 434,627 Int. Cl. D21c 3/20 US. Cl. 16272 4 Claims ABSTRACT OF THE DISCLOSURE Brightness reversion of low-cellulose wood pulps bleached with reductive bleaching agents is minimized by incorporation of a small amount of thiourea in the bleaching system.

This invention relates to the bleaching of wood pulps containing substantial amounts of non-cellulosic constituents, and particularly to the reductive bleaching of such relatively impure wood pulps by a method which provides bleached pulps resistant to color reversion upon aging.

Wood pulps fall generally into two principal classes. One is the so-called purified, or chemical, pulps which are prepared by pulping methods involving chemical removal of much of the non-cellulose wood materials, such as lignins and other impurities, to leave a relatively purified pulp composed of on the order of 80 to 100% of cellulose. These pulping methods involve a chemical digestion, and are typified by the kraft, sulfite and the like methods.

The other principal class of wood pulps is prepared by methods which leave the bulk of the non-cellulosic constituents of the wood in the pulp. These methods normally are referred to as mechanical and chemi-mechanical methods, in which the separation of the wood material into fibers operates primarily through mechanical attrition of the wood, either in the form of chips or as logs. Avoidance of strong chemical attack on the wood material leaves the bulk of the non-cellulosic constituents in the fibers. Such fibers contain up to about 60% of cellulose, with the remaining elements in the pulp being on the order of 40 to 60% of non-cellulose wood materials.

The strictly mechanical methods for producing pulps are represented by the so-called groundwood process, in which logs or other large pieces of wood are ground on a grinding stone, and the refining methods in which the wood chips are mechanically subdivided in disc or the like refiners. Chemi-mechanical methods involve softening of the wood with aqueous softening agents such as sulfites, bisulfites and the like (without substantial extraction of non-cellulosics) preliminary to mechanical subdivision of the wood into fibers. Pulps produced by either of these essentially mechanical means are referred to herein as low-cellulose wood pulps.

The low-cellulose wood pulps are particularly desired because of their low cost and generally satisfactory phys ical properties. Their preparation involves very little loss of the original wood, and methods of producing them are generally less expensive to operate than are the so-called chemical methods. The particular utility of these pulps is in the preparation of printing papers, newsprint, molded products, corrugated paper, boxboards and the like.

While retention of the bulk of the Wood materials in mechanical pulps is favorable from an economic point of view, however, it gives rise to a dark color in the pulps, and makes it difficult to bleach them. As a result of diligent efforts in the art, reductive bleaching methods employing hydrosulfites, borohydrides, thiourea dioxide 3,507,743 Patented Apr. 21, 1970 and the like have been devised which are effective to bleach these difficultly bleached pulps. Even with the best of these methods, however, the bleached pulps tend to revert toward their original dark color on standing, particularly in the wet pulp form, and it has been a major goal of the trade to overcome this reversion problem.

It is a feature of our invention to provide a method for minimizing the color reversion encountered in reductively bleached low-cellulose pulps.

It is a further feature of our invention to provide such a method which does not alter the basic simplicity of the reductive bleaching processes, and which operates without substantial added expense.

We have now found that by carrying out reductive pulp bleaching processes on low-cellulose wood pulps in the presence of about 0.05 to 1.0%, and preferably about 0.10 to 0.20%, by weight of thiourea based on the weight of the pulp, color reversion on aging of the bleached pulp provided by the treatment is minimized. The reductive bleaching processes are carried out under bleaching conditions at a pH within the range of 2.0 to 8.5 and at a temperature within the range of about to 300 F., and preferably about to 212 F., for up to about 4 hours, and preferably about 0.5 to 1 hour, and normally are carried out in an aqueous pulp slurry containing about 1 to 20%, and preferably about 3%, by weight of the pulp and a bleaching amount of a reductive bleaching agent or mixture of reductive bleaching agents, suitably but not exclusively a hydrosulfite such as sodium or zinc hydrosulfite, thiourea dioxide, or a mixture of an alkali metal borohydride and bisulfite ion. The bleaching amount of reductive bleach is normally within the range of 0.01 to 2.0% based on the dry pulp weight, and will be determined for the particular reductive bleach used. A useful thiourea dioxide method is taught in US. Patent No. 3,384,534, issued May 21, 1968, co-pending patent application Ser. No. 418,911, filed Dec. 16, 1964 and assigned to the assignee of this invention, while a useful. borohydride pulp bleaching method is taught in US. Patent No. 3,284,283, issued Nov. 8, 1966, co-pending patent application Ser. No. 345,119 filed Feb. 17, 1964, and also assigned to the assignee of this invention.

The thiourea employed in the process of this invention to minimize color reversion in reductive bleaching processes has the following formula:

It is introduced into the herein bleaching system either as the solid or dissolved in water or bleaching solution. It can be introduced either prior to or simultaneous with introduction of bleach chemicals. Preferably the thiourea is introduced prior to addition of bleaching chemicals. The thiourea is used in the amount of about 0.05 to 1.0% by weight based on the weight of dry pulp. Preferably 0.1 to 0.2% is used.

The pulps which are treated by the herein process are the low-cellulose wood pulps, a term which is used herein to define wood pulps containing not more than about 60% by weight of cellulose, with the remaining material being non-cellulose 'wood constituents. These low-cellulose wood pulps are produced from a variety of wood sources, for example, but not exclusively, spruce, fir, hemlock, pine, cottonwood, aspen and poplar.

The low-cellulose wood pulps are treated in the present process in the form of an aqueous slurry having a concentration of about 1 to 20% by Weight, and preferably of about 3% by weight. Treatment of slurries containing substantially less than 1% of the pulp is uneconomical, requiring excessive equipment investment, while treatment of pulps having concentrations higher than about 20% is difficult because of the mechanical problems associated with providing good mixing of the treating chemicals with the pulp.

The thiourea additive of this invention improves the color reversion properties of bleached pulps prepared with any of the reductive bleaching agents. Typical reductive bleaching agents are sodium or zinc hydrosulfite, thiourea dioxide and a mixture of an alkali metal borohydride with bisulfite ion. The specific conditions employed with the various reducing bleaching agents vary; however, generally speaking they all employ temperatures within the range of about 90 to 300 F., they operate at a pH within the range of about 2.0 to 8.5, and are carried out over a period of up to about 4 hours. The thiourea is employed with them in the amount of about 0.05 to 1.0%, and preferaibly about 0.1 to 0.2%, based on the weight of the pu p.

A particularly useful reductive bleaching process employing thiourea dioxide is described fully in co-pending patent applications Ser. No. 418,911, assigned to the assignee of this invention. In brief, this process involves treating the low-cellulose wood pulp at about 90 to 300 F., and preferably 90 to 212 F., for up to 4 hours at a pH of about 5.5 to 8.0 in the presence of about 0.1 to 2.0% of thiourea dioxide on the weight of the pulp. The effectiveness of the thiourea dioxide is improved by addition of sodium bisulfite in an amount up to 20 times the amount of weight of thiourea dioxide employed, and/or by addition of polyphosphate, for example sodium tripolyphosphate, in amounts of up to 1% by weight of the Weight of the pulp.

A very useful process employing an alkali metal borohydride and bisulfite ion is taught fully in co-pending patent application Ser. No. 345,119, assigned to the assignee of this invention. In this preferred process of employing borohydrides in reductive bleaching, the cellulosic fibers are impregnated with an aqueous solution containing bisulfite ion and having a pH of 2 to 7 to provide at least about 0.04% of the bisulfite ion on the fibers, contacting the resulting bisulfite ion-impregnated fibers with 0.01 to 0.5% of the weight of the fibers of an alkali metal borohydride, and permitting the reagents to act on the fibers to increase their brightness. This process is run at a temperature of about 90 to 300 F.

The reductive bleaching of pulp may also be carried out with sodium hydrosulfite, by a process in which pulp having a concentration of about 1 to 20% by weight is treated in aqueous slurry form with sodium or zinc hydrosulfite (Na S O or ZnS O employed in the amount of about 0.2 to 1.5% of the Weight of the pulp, at about 90 to 212 F. and a pH in the range of about 4.5 to 8.5 for up to 4 hours.

The pH of the aqueous phase of the pulp slurry during the useful reductive bleaching processes ranges from about 2 to 8.5, with the specific range for a given bleaching agent varying within these range limits. As indicated above, the borohydride-bisulfite method is operated at a pH of about 2 to 7, while the thiourea dioxide process preferably is operated at a pH of about 5.5 to 8.0 The sodium hydrosulfite process on the other hand is operated at a pH of about 4.5 to 8.5. To provide these pHs, where necessary alkalies such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate or the like are provided in the aqueous phase of the pulp slurry. Amounts of alkali on the order of up to about 0.2% by weight in the solution, or even sometimes as high as 0.4 or 0.5% by weight, are employed to provide the proper pH where required.

In essentially all reductive bleaching processes addition of sodium tripolyphosphate or other polyphosphate and/ or metal chelating agents such as diethylene triamine pentaacetic acid, ethylene diamine tetraacetic acid, and the like, materially improve the bleaching effect of the reductive bleach. Addition of sodium bisulfite also improves the bleach in the thiourea dioxide process, and in the borohydride process it co-acts with the borohydride to provide bleaching. Polyphosphates when employed normally are used in the amount of up to about 1% by weight based on the dry weight of the pulp, and chelating agents are used in amounts up to about 0.5%. Typical useful polyphosphates are sodium tripolyphosphate, tetrasodium pyrophosphate, sodium hexametaphosphate and the like, with potassium polyphosphates being interchangeable with the sodium polyphosphates for all practical purposes.

The bleaching process preferably is carried out by slurrying the pulp in water at the desired concentration, and adding the bleaching agents into the pulp slurry. Mild agitation is provided initially to provide even distribution of bleaching chemicals. The temperature is raised to the desired level by external heating means, by passing steam through the pulp slurry, or the like. In small scale operation it is sometimes desirable to carry out the process in an inert non-oxidizing atmosphere, for example by employing a closed vessel with a nitrogen cover. In large scale operation the bulk of the process materials minimizes the oxidative effect of the air with which the system is in contact. Following the bleaching treatment, if desired, although not necessarily, the pulp is washed with water and ether dried or used directly in producing end product.

In the following examples, which are presented only by way of illustration of this invention and are not to be considered as limiting the scope thereof in any way, pulp brightnesses were determined on handsheets formed in a B'tichner funnel by TAPPI Test Method T218M-59. The brightnesses of the handsheets were determined on a. Gardner Reflectometer corrected to G.E. Standards, in accordance with TAPPI Test Method T217M-48.

The susceptibility of the bleached wet pulps to color reversion was determined by an oxidation treatment involving passing oxygen gas through 3 g. pulp samples. The samples were at a 2.0% oven dry pulp concentration, and 750 ml. of oxygen gas was passed through the samples in 3 minutes while the pulp temperature was held at F. The brightness reversions, otherwise expressed as drops in GE. brightness values, produced by this test correlate closely with the behavior of pulps bleached by the procedures of the examples when such pulps are handled by customary transfer, mixing and other procedures over the usual times, e.g. 4 to 20 hours, in pulp from during manufacture of articles from the pulp.

EXAMPLE 1 Two samples of a West Coast pine refiner groundwood, having an unbleached G.E. brightness of 50.3% were slurn'ed with boiling dilution water in a slow speed paddle mixer to a concentration slightly higher than 3% by Weight on an oven-dry pulp basis, such that subsequent addition of bleaching and other treating chemicals provided a final pulp concentration of 3% by weight. Five tenths percent by weight based on the weight of the pulps of sodium tripolyphosphate was then mixed into each of the slurries in a Waring blender, and the resulting pulps were transferred into polypropylene bottles, and purged of air with compressed nitrogen gas. One-tenth percent of thiourea based on the weight of the pulp, and then 1.25% by weight of sodium hydrosulfite based on the weight of the pulp, were introduced into the first bottle, and the bottle capped. Only the sodium hydrosulfite was introduced into the second. The pulp slurries, both at a pH of about 6.0, then were thoroughly mixed by shaking the bottles and they were placed in a temperature controlled water bath for four hours at 160 F.

Following the treatment the pulps were subjected to the oxidation test referred to above, and then tested for brightness, which proved to be 65.9% in the first sample which was bleached in the presence of the thiourea additive of this invention. The second sample, bleached by a system not containing the thiourea, had a brightness of only 62.8%.

EXAMPLE 2 The procedure of Example 1 was followed with the exception that the Example 1 reductive bleaching system of sodium hydrosulfite and sodium tripolyphosphate was replaced with 0.75% by weight of thiourea dioxide, 0.2% of sodium hydroxide, and 0.5% of sodium tripolyphosphate, all based on the weight of the pulp. One-tenth percent by weight of the thiourea on the weight of the pulp was employed in one sample and no thiourea was used in the other. The pH of the slurries during bleaching was about 6.0.

The brightness of the pulp sample bleached in the presence of thiourea as an additive in accordance with this invention was 60.7%, whereas the brightness of the corresponding pulp bleached without the thiourea was 58.8%; both pulps were tested after the above-described oxidation treatment of the bleached pulp.

EXAMPLE 3 The procedure of Example 1 was followed with the exception that the sodium hydrosulfide-sodium tripolyphosphate bleaching system of Example 1 was replaced with 0.05% by weight of sodium borohydride, 3.1% by weight of sodium bisulfite, and 0.5% by weight of sodium tripolyphosphate, all based on the weight of the pulp. Onetenth percent by weight of thiourea on the weight of the pulp was employed in one sample and no thiourea was used in the other. The pH of the slurries during bleaching was. about 6.0. The pulp bleached with the system containing the thiourea additive of this invention was 63.2% following the oxidation treatment described above, whereas the brightness of the pulp bleached without the thiourea was only 60.4% following the oxidation treatment.

EXAMPLE 4 The procedure of Example 1 was followed with the exception that the pulp treated was a northern fir-balsam stone groundwood pulp having an initial brightness of 60.2%, and the sodium hydrosulfite-sodium tripolyphosphate bleaching system employed in Example 1 was replaced with sodium borohydride in the amount of 0.1%, sodium bisulfite in the amount of 1.1%, and sodium tripolyphosphate in the amount 0.5% all based on the weight of the pulp. One-tenth percent of thiourea was employed in one sample and no thiourea was used in the other. The pH of the slurries during bleaching was about 5.5 to 6.0.

In the bleach conducted with the thiourea additive present in accordance with this invention, the brightness of the pulp following oxidation by the above procedure was 71.4%. The bleach not involving the use of thiourea produced a pulp which following oxidation had a brightness of only 69.5%.

EXAMPLE 5 The procedure of Example 1 was followed with the exceptions that the pulp treated was northern pine stone groundwood having an initial brightness of 55.9%, and that in place of the sodium hydrosulfite-sodium tripolyphosphate bleaching system, 0.5 of thioureau dioxide, 3.3% of sodium bisulfite, 0.5 of sodium hydroxide and 0.5% of sodium tripolyphosphate, all based on the weight of the pulp, were employed as the bleaching system. Thiourea was employed in the amount of 0.1% based on the weight of the pulp with one sample, and no thiourea was used in the other. The pH of the pulp slurries during bleaching was about 7.0, and they were maintained at 160 F. for three hours rather than 4 hours as in Example 1.

The brightness of the pulp bleached with the system containing the thiourea additive of this invention had a brightness of 65.6% following the oxidation treatment described above, whereas the pulp bleached with the system not containing thiourea had a brightness of only 63.8% after the oxidation treatment.

6 EXAMPLE 6 Two West Coast fir refiner groundwood pulp samples having an unbleached brightness of 52.6% were slurried with boiling dilution water in a Waring blender. The pulp slurries were then transferred into stainless steel containers, placed in an oil bath controlled at 257 F. to 311 F., and the slurries were heated to 190 to 212 F. Since the system was operated under steam-generating conditions, a nitrogen purge was not used.

One and twenty-five hundredths percent by weight of sodium hydrosulfite and 0.5 by weight of sodium tripolyphosphate was added to each of the pulps with stirring. In one sample thiourea was also employed in the amount of 0.1%, being added just prior to addition of the sodium hydrosulfite; no thiourea was used in the other sample. Following addition of all chemicals, over a period of about one-half minute, the pulp was held at 200 to 212 F. with stirring at 2% pulp concentration for 5 minutes at a pH of about 6.0. The samples were then processed as described above, with the pulp bleached in the presence of thiourea having a brightness of 63.8% following the oxidation treatment, and the pulp bleached in the absence of thiourea having a brightness of only 62.0%.

EXAMPLE 7 Eifect of thiourea concentration The procedure of Example 1 was followed to bleach a series of pulp samples in the presence of varying amounts of thiourea. The pulp bleached was West Coast fir refiner groundwood having an unbleached brightness of 52.6%, and the bleaching chemicals were sodium hydrosulfite in the amount of 1.25%, and sodium tripolyphosphate in the amount of 0.5%, by weight based on the weight of the pulp. A series of samples was made, the first containing no thiourea, and five others containing thiourea in the amounts shown in Table 1 which follows. When used the thiourea was added to the pulp prior to introduction of the sodium hydrosulfite and sodium tripolyphosphate. The pH of the slurries during bleaching was about 5.7 to 6.0, and the pulps were mtaintained at F. for 3.5 hours. The results of the series of bleaches are shown in Table 1 which follows:

1 Based on oven dry pulp weight. 2 Sodium hydrosulfite. Sodium Tripalyphosphate.

4 Following bleaching and oxidation treatment.

The above examples show the improvement in brightness retention provided in reductively bleached pulps when thiourea is present during bleaching. A like effect is obwhen the thiourea is added shortly after addition of the bleaching chemicals, but before bleaching has occurred and reversion has begun to set in.

The improvement in brightness retention represents a real advantage in the bleaching of low-cellulose wood pulps, making it possible to use the pulps directly in applications in which such pulps could not heretofore be employed.

Pursuant to the requirements of the patent statutes, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art, such exemplification including what is considered to represent the best embodiment of the invention.

What is claimed is:

1. In the process of reductively bleaching low-cellulose wood pulps, wherein a low-cellulose wood pulp is bleached in an aqueous slurry containing 1 to 20% by weight of said pulp, and on the weight of said pulp 0.05 to 0.5% of an alkali metal borohydride and at least 0.04% of bisulfite ion, at a pH of 2 to 7 and a temperature of 90 to 300 F., for up to four hours, the improvement of including in said pulp with said borohydride and said bisulfite ion, 0.05 to 1.0% of thiourea based on the Weight of said pulp, whereby brightness reversion of the bleached pulp produced by said reductive bleaching process is minimized.

2. The process of claim 1 in which the thiourea is present in the amount of 0.1 to 0.2% based on the weight of the pulp.

3. In the process of reductively bleaching low-cellulose wood pulps, wherein a low-cellulose wood pulp is bleached in an aqueous slurry containing 1 to 20% by weight of said pulp, and on the weight of said pulp 0.1 to 2% of thiourea dioxide at a pH of 5.5 to 8.0 and a temperature of 90 to 300 F., for up to four hours, the improvement of including in said pulp with said thiourea dioxide 0; 05 to 1.0% of thiourea based on the Weight of said pulp, whereby brightness reversion of the bleached pulp produced by said reductive bleaching process is minimized.

4. The process of claim 3 in which the thiourea is present in the amount of 0.1% to 0.2% based on the weight of the pulp.

References Cited UNITED STATES PATENTS 2,071,304 2/1937 Hirschkind 16272 X HOWARD R. CAINE, Primary Examiner US. Cl. X.R. 16280 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,507,7 8 Dated April 21, 1970 Inventor(s) Louis P. Cartsunis and Rc bert R. Kindr'on It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 55, "oven-dry" should read --over-dr'y-.

Column 5, line 61, "thiour-eau" should read -thiou15ea-.

Column 6, line 59, "ob-" should read --obtained--.

Signed and sealed this 214th day of August 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents FORM PO-IDSO (IO-69) USCOMM-DC SOSTG-PGD n 14.5 r-ovnuuurr rnuu'lmz OFFICE: nu 0-366-334 

